The effect of weld-line reduction by using the laser metal sintered mold and the effect of hot air supply on the mold surface just before the injection have been investigated. The depth of the weld-line groove in the laser metal sintered mold is smaller than that in the conventional steel mold. Mold pressure increase in the laser metal sintered mold is faster than that in the steel mold and the peak pressure in the laser metal sintered mold is higher than that in the steel mold. This must be due to the low thermal conductivity of the laser metal sintered mold. As a preheating effect of the mold surface before the injection, hot air has been supplied on the mold surface using an air jet bulb. The reduction of the weld-line groove has been promoted in the laser metal sintered mold whereas few effects appeared in the steel mold. Surface temperature increase by the hot air supply in the laser metal sintered mold is larger than that in the steel mold. By this higher temperature increase, temperature of the resin on the weld-line area must be higher and the hardness of the resin must be lower in the laser metal sintered mold so that the resin is deformed well reducing the weld-line groove.
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